Since Alli hit shelves in June, two million starter kits of this first over-the-counter weight loss pill were sold in the five months, GlaxoSmithKline said.

Alli is the only nonprescription weight loss drug approved by the Food and Drug administration. In clinical trials, the FDA said that people using Alli lost an additional 2 to 3 pounds for every 5 pounds lost through diet and exercise. When taken with meals, the drug blocks the absorption of about one-quarter of any fat consumed. That fat -- about 150 to 200 calories worth -- is passed out of the body, potentially resulting in loose stools.

Malesia Dunn— a company spokeswoman— said, "Clearly given the overweight and obesity numbers in this country, Alli can have a long-term, sustainable business."

On the other hand, “While it's too early to tell how many consumers will continue using the drug, feedback on the drug's Web site so far has been overwhelmingly positive”, said Joe Cadle— marketing director for Glaxo Consumer Healthcare. The pharmaceutical company expects to sell between 5 million and 6 million kits annually, translating to at least $1.5 billion in annual retail sales.

Two-thirds of the U.S. population is estimated to be overweight or obese. A body mass index over 30 is considered obese. Labeling indicates Alli is appropriate for anybody who is overweight, or has a body mass index of 25 or higher.

Glaxo is banking on Alli to become a major drug in its portfolio. The company is spending $150 million on marketing the drug this year, making it one of the drug maker's biggest campaigns to date.

Alli comes in "starter kits" containing a food journal, a healthy-eating guide and a fat and calorie reference guide. A 60-capsule kit costs about $50 while a 90-capsule pack costs about $60.

Analysts say Alli's long-term success will hinge on Glaxo's ability to convince people that diet pills aren't a magic bullet. Failing to do so could lead to disappointment and poor sales.

Steve Brozak— an analyst with WBB Securities— said, "If the marketing is done correctly, it has to be a long and drawn out campaign."

Glaxo's understated campaign so far is a good sign, Brozak said. But he added that there's no way to truly gauge the financial success of the drug so soon after its release.

However, the major drawback encountered was the drug's unpleasant side effects, such as leakages and oily discharges. About half of patients in clinical trials for Alli experienced similar gastrointestinal side effects.

On the other hand GlaxoSmithKline is frank about the side effects, which it says can be controlled if the drug is used properly. The campaign stresses the importance of keeping meals under 15 grams of fat to avoid effects.